1. Field of the Invention
The present invention relates to an apparatus and method for continuously forming a non-single-crystal semiconductor thin film and, for example, to an apparatus and method for mass-producing photovoltaic devices of large area such as photovoltaic devices of amorphous silicon or an amorphous silicon alloy. More particularly, the invention concerns controlling means for controlling the temperature of walls of a film-forming chamber and the introduction of a film-forming gas so as to obtain a functional deposited film with high quality.
2. Related Background Art
In recent years, research and development has been conducted and is under way for electric power generation methods with solar cells utilizing the sunlight, as clean power generation methods that can support the increase in demand for power in the future without causing environmental destruction, because they do not pose the problems of contamination with radioactive emissions, global warming, and so on, because the sunlight comes down all over the earth and thus the energy source is not localized, and because relatively high power generation efficiency is achieved without necessitating complex and large-scale facilities.
For the electric power generation methods using solar cells to meet the demand for power, they have to satisfy fundamental requirements. For example, the solar cells used must have sufficiently high photoelectric conversion efficiency, must have excellent stability of characteristic, and must be mass-producible.
Under such circumstances, the solar cells drawing attention are those produced by using a readily available source gas such as silane and decomposing it under glow discharge, thereby depositing a semiconductor thin film of amorphous silicon (hereinafter referred to as "a-Si") or the like on a relatively cheap substrate such as glass, a metal sheet, or the like. The solar cells produced using amorphous silicon are readily mass-producible and because they have the possibility of being produced at lower cost than the solar cells produced using single-crystal silicon or the like. A variety of proposals have been made heretofore on their basic structures, production methods, and so on.
The following technology is known as a conventional method for forming the photovoltaic device.
For example, fabrication of photovoltaic devices using non-single-crystal semiconductor films or the like has been conducted by using the plasma CVD process and has been industrialized. However, the photovoltaic devices are required to have sufficiently high photoelectric conversion efficiency, excellent stability of characteristic and mass-producibility.
To meet theses requirements, fabrication of the photovoltaic devices using the non-single-crystal semiconductor films or the like must achieve improvements in electrical, optical, photoconductive, or mechanical characteristics as well as in fatigue characteristics or operating environment characteristics in repetitive use. Such fabrication must allow mass production with repeatability by high-speed film formation while also achieving an increase of film area and uniformity of film thickness and film quality. It is thus pointed out that these are problems to be solved in the future.
Many electric power generation methods using the photovoltaic devices employ a method for connecting unit modules in series or in parallel to form a unit, thereby obtaining desired electric current and voltage. In that case each module is required to be free of wire breaking and short circuits. In addition, an important requirement is that there are no variations in output voltage and output current among the modules.
The characteristic uniformity of the semiconductor layers is the most important factor for determining characteristics of the unit module, at least in the stage of forming each unit module. From the viewpoints of facilitating module designing and simplifying module assembling steps, providing a semiconductor deposited film excellent in the characteristic uniformity over a large area is required in order to enhance mass producibility of photovoltaic devices and to achieve great reduction of production cost.
Proposed as an efficient mass production method of photovoltaic devices is a method for producing an amorphous-silicon-based solar cell, in which independent film-forming chambers for formation of the respective semiconductor layers are provided and each semiconductor layer is formed in each film-forming chamber therefor.
Incidentally, U.S. Pat. No. 4,400,409 discloses a continuous plasma CVD system employing the roll-to-roll method.
This system can continuously fabricate the device having a semiconductor junction by providing a plurality of glow discharge regions, setting a sufficiently long, flexible substrate of a desired width, and continuously conveying the substrate in the longitudinal direction thereof while depositing the semiconductor layers of the desired conduction types in the respective glow discharge regions.
In the U.S. patent, gas gates are used in order to prevent the dopant gas used upon formation of each semiconductor layer from diffusing or mixing into other glow discharge regions.
Specifically, the system employs means for separating the glow discharge regions from each other by a slit-shaped separation passage and for forming a flow of scavenging gas, for example Ar, H.sub.2, or the like, in the separation passage. Therefore, this roll-to-roll method is a method suitable for mass production of the semiconductor device.
However, the formation of each semiconductor layer is carried out by the plasma CVD process using RF (radio frequency) and thus there is a limit to increasing the film deposition rates while maintaining the characteristics of films continuously formed.
Specifically, for example, even in the case where a semiconductor layer is formed in the thickness of at most 2000 .ANG., there does exist a method for always inducing a predetermined plasma throughout a considerably long and large area and maintaining the plasma uniform.
Skills are, however, necessary for this method and it is thus rather difficult to generalize various plasma control parameters.
In addition, decomposition efficiency and utilization-efficiency of source gases used for formation of film is not so high, which is one of the causes of raising the production cost.
On the other hand, the plasma process using the microwave is drawing attention recently. Since the frequencies of microwaves are short, the energy density can be increased more than in the conventional cases using RF. Microwaves are thus suitable for efficient generation and continuation of plasma.
For example, U.S. Pat. No. 4,729,341 discloses a low-pressure microwave CVD process and system for depositing a photoconductive semiconductor thin film on a large-area cylindrical substrate by a high-power process using a pair of radiation type waveguide applicators.
Taking the above circumstances into consideration, a mass-producing method of higher throughput can be obtained by rationally combining the microwave plasma CVD process (hereinafter referred to as ".mu.W-CVD process") with the roll-to-roll production method, both being said to be suitable for mass production.
There was, however, the following problem in the roll-to-roll .mu.W-CVD process.
The problem is that the input microwave power is not used only for decomposition of source gases for forming a deposited film, but also the microwave power indirectly heats the wall of the deposition chamber forming the film-forming space through the high plasma density, or the microwave itself directly heats the wall to high temperature. The temperature of the wall of deposition chamber starts increasing at the same time as input of microwave power; after a certain period of time has passed, it reaches the terminal temperature determined by a discharge power value or the like. It may reach 350.degree. C. or even about 450.degree. C. depending upon the conditions.
As a result, there arose the following problems.
The first problem is that the temperature of the beltlike substrate for film deposition increases due to the high temperature of the wall of deposition chamber, so that the substrate temperature cannot be maintained around 250.degree. C., which is normally considered to form a deposited film with good quality.
Solar cells fabricated under such circumstances will have low conversion efficiency.
Further, the second problem is that, depending upon the material for the deposition chamber, the temperature could reach near the softening point of the material, whereby the wall of deposition chamber would be damaged.
Specifically, for example, when aluminum is used for the wall of deposition chamber, the temperature near 450.degree. C. will deform screwing portions, portions under tensile stress, etc. to make them of no use.
In order to prevent such an accident, a high-melting-point material should be selected or a cooling means for preventing the increase in the temperature of the wall of deposition chamber needs to be provided.
The third problem is as follows. In the case where a gas manifold for uniformly supplying the film-forming gas into the film-forming space is installed through a part of the wall of the deposition chamber, increase of temperature will also be seen in the gas manifold itself because of the increase in the temperature of the deposition chamber wall as described above. This promotes thermal decomposition of the film-forming gas in the gas manifold, thus greatly degrading initial characteristics of solar cells, and degrading the uniformity of characteristics, because of the temporal temperature increase in the gas manifold during the long-term formation of film.
In view of these problems, though the roll-to-roll .mu.W-CVD process is a method suitable for mass production of semiconductor devices, the further improvements in the photoelectric conversion efficiency, characteristic stability, and characteristic uniformity, and the reduction of production cost are necessary for widespread and volume use of the photovoltaic device, as described previously.
Particularly, for the improvements in the photoelectric conversion efficiency and characteristic stability, each unit module should be preferably produced with as high photoelectric conversion efficiency and as low characteristic deterioration rate as possible.
Further, when the unit modules are connected in series or in parallel to form a unit, a unit module having the minimum current or voltage among the unit modules constituting the unit limits and determines the characteristics of the unit. Therefore, it is very important to decrease the characteristic variation of the unit modules, as well as improving the average characteristics of each unit module. It is thus demanded that the characteristic uniformity of the semiconductor layers themselves which is the most important factor for determining the characteristics of the unit modules, be secured in the stage of forming the unit modules.
It is also strongly demanded that the yield be increased by decreasing defects of the semiconductor layers so as to avoid the breaking of wires and short circuits in each module, to decrease the production cost.